date. Larval age was estimated using photomicro- 

 graphs of larvae of known age and size for com- 

 parison, available in Chanley and Andrews (1971). 

 The initial occurrence of larvae in each year was 

 dominated by early straight hinge larvae. The 

 spawning season was defined by larval abundances 

 >10 m~ 3 . This level was chosen arbitrarily to 

 distinguish major spawning from occasional low lar- 

 val abundances (<10 m~ 3 ). 



Environmental variables that were examined for 

 correlations with the initiation of spawning and lar- 

 val abundance included water temperature, phyto- 

 plankton abundance, degree days, calendar date, 

 and lunar cycles. Water temperatures were taken 

 concurrently with the plankton samples. Phyto- 

 plankton abundances from July 1974 to August 1977 

 were available for the Damariscotta River (McAlice 

 unpubl. data). Data from the neighboring Sheepscot 

 River estuary (McAlice and Denniston 3 ) were sub- 

 stituted for the period October 1969 to June 1974. 

 The decision to use the Sheepscot data was based 

 on the highly significant Spearman's rank correla- 

 tion (Zar 1984) (r = 0.67, P < 0.001) between the 

 Damariscotta and Sheepscot phytoplankton abun- 

 dances from July 1974 to August 1977. Degree days 

 were calculated in the manner described by Thiesen 

 (1973). For each year, degree days were summed 

 from the time of peak larval abundance the previous 

 year to the initiation of spawning. Lunar cycle in- 

 formation was obtained from tide tables published 

 by NOAA (1969-76). 



once initiated, probably continued throughout the 

 summer as indicated by the persistence of early 

 stage mussel larvae. Spawning appeared to cease 

 as temperatures fell to 9° -14° C in September and 

 October (Fig. 2), when only late stage larvae were 

 present. Maximal larval abundances were observed 

 in mid- to late June, shortly after spawning began. 

 At this time, straight hinge larvae, <6 d old, were 

 dominant. Maximum values for the period 1970-75 

 ranged from 787 larvae m~ 3 to 5,400 larvae m~ 3 . 

 In 1976 and 1977, maximum abundances were an 

 order of magnitude larger (3.16 x 10 4 m~ 3 and 

 6.09 x 10 4 m -3 , respectively). Following the peaks 

 in June, larval densities generally declined through 

 1 to 3 successively smaller peaks (Fig. 2). 



Mussel larvae appeared well after phytoplankton 

 abundances had begun to increase from low winter 

 values to generally high summer values (Fig. 3). Lar- 

 vae usually disappeared before phytoplankton abun- 

 dances fell to typically low winter levels. 



In addition to the larvae of Mytilus edulis, those 

 of Anomia simplex, Geukensia demissa, Modiolus 

 modiolus, and what was probably a complex of My a 

 arenaria, Hiatella arctica, and possibly Sphenia 

 sincera (Hanks and Packer 1985) larvae were also 

 identified. Amonia simplex occurred most commonly 

 from September through December, though never 

 in great numbers. The Mya-Hiatella-Sphenia group 

 was often very abundant, and occurred from early 

 May through September. Geukensia and Modiolus 

 were never common. 



Results 



Examination of the age and abundance of mussel 

 larvae from December 1969 to September 1977 in- 

 dicated that spawning began in late May or early 

 June when temperatures reached 10° -12.5°C (Fig. 

 2). The average date when spawning began was 4 

 June, with a standard deviation of approximately 

 7 d. The average number of degree days prior to 

 spawning was 2,853, with a standard deviation of 

 368. No significant relationship was found between 

 degree days and commencement of spawning or 

 degree days and maximum larval abundances. 



Commencement of spawning may be related to the 

 time of spring tides (Table 1). In 7 of the 8 yr ex- 

 amined, spawning began within 5 d, before and 

 after, a spring tide. On four occasions spawning 

 commenced within 2 d of a spring tide. Spawning, 



3 McAlice, B. J., and F. D. Denniston. Dominance and diversity 

 of Sheepscot River estuary phytoplankton. Manuscr. in prep. 

 Ira C. Darling Center, University of Maine, Walpole, ME 

 04573. 



Table 1 .—Estimated dates and temperatures of the initiation and 

 cessation of spawning for Mytilus edulis in the Damariscotta River 

 estuary, and dates of nearest spring tides, 1970-77. 



Discussion 



A temperature threshold for spawning was in- 

 dicated by the appearance of Mytilus larvae when 

 water temperatures exceeded 10°-12.5°C and the 

 subsequent disappearance of larvae when tempera- 

 tures fell below 9° -14°C. A number of studies have 

 reported the initiation of spawning in Mytilus edulis 



997 



